Sealing arrangements are used in fluid swivels. Such fluid swivels are used in offshore installations to transfer fluids between fixed underwater pipe lines and floating vessels which may weathervane around earth-fixed installations. Fluids can be withdrawn from a well head or can be introduced or can be loaded/offloaded or used for power and control of equipment.
A typical fluid swivel includes at least two swivel members comprising ring-shaped inner and outer walls forming an annular chamber between them and forming cylindrical clearances at each side of the chamber between the fixed and rotating wall members. Several circular grooves are provided in the cylindrical clearances to hold an arrangement of piston or rod seals at each side of the fluid chamber.
In order to transfer a large number of separate fluid lines of small to large pipe diameter sizes, several swivel modules are assembled together in a stack. The lowest swivel path of a swivel assembly requires a relatively large sealing diameter to provide sufficient space in the centre of its inner ring to guide the entry piping of the above situated swivels. This sealing diameter of the lowest swivel path increases with the total fluid throughput of a complete swivel assembly.
The first point at which problems occur during increasing of the pressure is at the sealing of the clearance between the two swivel members. The seal grooves and the clearances widen as the fluid pressure increases, because the inner and outer walls of the swivel are driven apart by the fluid pressure. The growth of seal groove widths and the clearances may lead to the allowable extrusion gap size of the seals being exceeded and consequently result in extrusion of the seal in the extrusion gap and/or in cracking of the seals.
It is possible to address the extrusion problem by decreasing the initial clearance between the two members in the none-pressurized condition and by making the inner and outer swivel rings sufficiently rigid. However, this will result in a risk of interference during differential temperatures of the inner and outer parts, a sharp increase of material costs because the stiffening results in very massive swivels, which jeopardises the feasibility particularly when a large sealing diameter is required.
From U.S. Pat. No. 4,828,292 and U.S. Pat. No. 4,925,219 a sealing assembly is known wherein face seals are used. Face seals engage to opposite surfaces being parallel to the direction of displacement of the two swivel members and a surface of one of the swivel members, said surface being parallel to the axis, so that the face seals are oriented between flat surfaces rather than cylindrical surfaces as piston or rod seals. The axial distance between the sealing surfaces remains essentially constant despite any radial deflection of the inner and outer swivel part caused by increase or decrease of fluid pressure, so that the extrusion gaps of the face seals are hardly affected by the fluid pressure. For this reason, swivel designs using these face seals do not require large wall section thicknesses to limit the radial deflections and can, therefore, be designed for strength rather than for stiffness.
Swivels based on face seals, however, require several inner and outer rings to create the grooves for their seals. These rings and the swivel bearing are to be machined with very high axial tolerances to limit the initial seal gap tolerances as result of the accumulated machining tolerances. The large number of swivel rings and the high machining tolerances result in high fabrication costs.